Tantalum (Ta) is one of the best hard masks for MTJ reactive ion etching (RIE) due to a very low etching rate using Methanol and high etching selectivity to MTJ materials. Usually, a Ta hard mask is patterned by a dielectric hard mask, for example silicon dioxide (SiO2) or silicon nitride (SiNx), and the dielectric hard mask is patterned by photoresist (PR). This is because there will be a fencing issue if PR is used directly to pattern Ta. However, SiO2 etching by high SiO2/PR selectivity chemical etching, e.g. C4F8, will generate severe striations around the SiO2 sidewall which will transfer to the Ta and even to the MTJ sidewall and lead to rough sidewalls and poor uniformity within the chip. Etching SiO2 by CF4 usually has much less sidewall striation; however, the poor SiO2/PR selectivity will limit the thickness of SiO2 and also Ta that can be etched with the same thickness PR.
Several patents show the use of various hard masks in etching MTJ stacks. These include U.S. Pat. No. 8,722,543 (Belen et al), U.S. Pat. No. 8,450,119 (Torng et al), U.S. Pat. No. 7,593,193 (Inomata et al), and U.S. Pat. No. 7,001,783 (Costrini et al). Other patents show passivating processes: U.S. Pat. No. 7,169,654 (Zia et al), U.S. Pat. No. 6,395,621 (Mizushima et al), U.S. Pat. No. 8,716,072 (Bangsaruntip et al), and U.S. Pat. No. 7,471,445 (Pan). The process of the present disclosure is not taught or suggested by any of these references.